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Review of Blasting Operations at Denver Brick Company's Golden Mine <br /> 2.3 Blast Noise Impacts <br /> The largest blast induced noise occurs at frequencies below the threshold of hearing for <br /> humans (16 to 20 Hz). Common industry term for blast-induced noise is "air-overpressure." It is <br /> the measurement of the transient pressure changes. These low intensity pulsating pressure <br /> changes, above and below ambient atmospheric pressure, are manifested in the form of <br /> acoustic waves traveling through the air. The speed of sound varies in different materials, <br /> depending on the density of the medium. For instance, pressure waves typically travel at the <br /> speed of 4,920 ft/s in water, whereas, air density is at only 1,100 ft/s because air has a lower <br /> density. <br /> When calculating maximum air overpressure values, the absolute value of the greatest pressure <br /> change is used, regardless of whether it is a positive or negative change.The frequency of the <br /> air overpressure (noise) is determined by measuring how pressure changes occur in one <br /> second of time. Blast noise occurs at a broad range of frequencies and the highest energy blast <br /> noise usually occurs at frequencies below that of human hearing (less than 20 Hz). <br /> 2.3.1 Air Overpressure Measurement Scales: <br /> Regular acoustical noise measurements taken for the purpose of monitoring compliance with <br /> local noise ordinances almost always use A-weighted (dBA)and C-weighted (dBC) scales. <br /> Instruments used for these A and C-scale measurements filter out most of the air overpressure <br /> occurring below a frequency of 20 Hz because humans cannot hear it and are generally not <br /> annoyed by it. Much of the air overpressure frequency spectrum created by rock blasting occurs <br /> at frequencies below 20 Hz.Accordingly, seismographs used for blasting measurements are <br /> equipped with microphones and recording equipment that captures all air overpressure <br /> fluctuations occurring from 2 to 200 Hz. These blasting measurements are called linear scale <br /> measurements and the unit is dBL. <br /> A significant amount of the energy in blast induced air pressure waves occurs at frequencies <br /> below 20 Hz. When A-weighted and C-weighted scales are used to record blast induced noise, <br /> much of the event is filtered out and the reported intensity or decibel values are significantly less <br /> than what would be recorded by a linear scale 2 Hz response microphone reporting results in <br /> dBL scale. Differences between decibel scale measurements for individual blasts will vary <br /> depending on their unique frequency intensity spectrums.The full range recording of blast <br /> induced noise can only be done with linear 2 Hz response instruments. <br /> In a study by the U.S. Bureau of Mines (RI 8485- Siskin et al, 1980), researchers measured <br /> blast induced noise at a common location using A-weighted, C-weighted and linear <br /> microphones. Comparable measurements taken about 800 feet from a blast, as shown in Figure <br /> 2.5, show that a linear peak noise of 120 dBL equates to only 112 dBC and 85 dBA. <br /> 12 <br />